Is this debt legally mine? HELP

My mother owned a spray tanning salon and the dba (doing business as) was in her name and my sister and I worked for her.To make a long story short she sent my sister and I to meet with a salesman to put an ad in a massive mail flyer for advertising, this was in 2004. He showed us the lay out and said he had a contract for us to sign I told him everything was in my mothers name and legally I didn't have the authority to sign the contract. He said just sign the contract and I'll fax everything over to your mother for her approval. Well you can guess what happened I signed it and put my SS# on it. He did faxed it over
and my mother didn't agree with the contract which locked us into 3 months of ads and the ad size. He was supposed to send an amended contract that they agreed to,but never did. Anyway we paid some of the invoice the part we agreed to, but not what the original contract said which left a balance of about $1200.00.
At the end of the 3 months salesman came into our place of business and said he would call it good if we agreed to give him some free tanning coupons which we did. About 2 months later we had to close down. I received a invoice for this bill at my house about 6 months ago and called and told them this wasn't my bill (I don't even know how they got my address). The county then changed my home address number so apparently all the invoices went back to them as undeliverable and my phone # is unlisted. So I get a call from my ex-husband yesterday (his phone# is listed) saying that this guy is a server and he is trying to serve me papers. My ex tells him he doesn't know where I live or my phone number, but I feel uneasy knowing they are looking for me. If I wasn't the owner of this business am I liable for this bill?
Also my sister was a witness to all of this.
I don't want this to go on my credit and can they put this on my credit?